An apparatus called a codec (often referred to as a video codec) has been proposed in a television conference system, a video-telephone system or the like, for example. The codec is used in order to encode and compress picture data upon transmission or recording thereof. How to encode the picture data was standardizcd by the video CODEC (coder and decoder) recommendation H. 261 approved by the Comite Consultatif International des Telegraphique et Telephonenique (CCITT) under the International Telecommunication Union (ITU) in December, 1990.
Dynamic-picture encoding is applied to broadcast communications, video conferencing, or the like, for example, in which a standard television camera may be used as a signal source and where the object is to transmit a signal to a far away place. It is also applicable when the goal is to process and store a signal locally.
In order to convert between several standard television formats such as PAL or NTSC, an intermediate video format was recommended by CCITT recommendation H. 261. This so called common intermediate format (CIF: Common Intermediate Format) can be used to account for the differences in television systems used in different regions of the world. For purposes of this document, CIF and Common Intermediate Format are intended to mean the format recommended by CCITT and similar intermediate formats which can be used for the same purpose. The differences between an NTSC system and a
system, for example, or the like are resolved by such intermediate formats, such that communication can be performed between CODECs. The CIF format has a picture resolution of 352 dots in the horizontal direction and 288 dots in the vertical direction, and a frame frequency which is that of 30/15/7.5 Hz.
In general, according to an encoding unit of the codec, input video data are encoded by an encoder and then multiplex-encoded. Further, after the data are temporarily stored in a transmission buffer, the data are encoded by a transmission encoder and transmitted as an encoded bit sequence. In a decoding unit thereof, the video data having the transmitted and encoded bit sequence is decoded by a transmission decoder, and the data are temporarily stored in a reception buffer and then multiplex-decoded. Further, the data are decoded to thereby obtain an original video signal.
In the case of transmitting a large quantity of picture data, video data are encoded and compressed upon transmission and subsequently decoded and decompressed upon reception. Accordingly, the codec can be used not only for transmission of the picture but also for the recording of the video data by a VTR, for example.
In case of a dynamic-picture codec, a circuit called a video pre-processor is connected to the preceding stage of an encoding unit of a portion standardized by the above-mentioned CCITT recommendation H. 261.
The video pre-processor is generally formed of a line interpolation circuit and a filter apparatus formed of a temporal filter and a motion adaptive type spatial filter. The line interpolation circuit repeats one line of video data every fifth line in order to convert video data of 240 lines (the NTSC system) into video data of 288 lines (the CIF). The temporal filter operates as a temporary cyclic type filter for improvement of compression efficiency as pre-processing upon compression of the picture. The motion adaptive type spatial filter performs the filtering so that motion in the picture becomes smoother whorl the picture is displayed on, for example, a television monitor or the like after transmission.
FIG. 4 shows an example of construction of an image pickup apparatus on the side of the coder of the above-mentioned codec, that is, on the hide for transmitting the video data.
In FIG. 4, reference numeral 2 represents a CCD (charge coupled device) of the NTSC or the PAL system for taking in a picture to be transmitted by a drive signal from a driving circuit 5. An analog video signal picked up and obtained by the CCD 2 is supplied to an analog processing circuit 3. After being subjected to various processing such as a blanking mixture, a black clip, a gain control, a pedestal adjustment, a shading correction, a gamma correction and so on, the analog video signal is supplied to an encoder (NTSC/PAL encoder) 4. After being encoded, the analog video signal is supplied to a decoder (NTSC/PAL decoder) 6 to be decoded thereby. An output of the decoder 6 is supplied to an A-D converter 7 to be converted into digital video data, which is then supplied to a CIF converting circuit 8. The CIF converting circuit 8 converts the 240 line video data, for example, into the above-mentioned video data of 288 lines by using a memory 9 and supplies video data obtained by the conversion to a eerier (a coder based on the recommendation H. 261) 10.
The coder 10 will be explained with reference to FIG. 5. FIG. 5 shows the inside construction of a video codec (picture compressing unit) mounted on a dynamic-picture codec as disclosed in Japanese patent application number 4-195615 which is hereby incorporated by reference. In FIG. 5, the coder 10 shown in FIG. 4 corresponds to an encoding controlling circuit 14, an information-source encoder 15, a video-signal multiplexing encoder 16, a transmission buffer 17 and a transmission encoder 18. A transmission decoder 21, a reception buffer 22, a video-signal multiplexing decoder 23 and an information-source decoder 24 form a decoder.
As shown in FIG. 5, in a video codec 11, video data supplied through an input terminal 13 from a signal source (transmission side or the like) not shown are subjected to encoding processing by the information-source encoder 15 on the basis of control of a controlling signal supplied from a controlling unit not shown through an input terminal 12 and the encoding controlling circuit 14. The data subjected to the encoding processing arc subjected to encoding processing by die video-signal multiplexing encoder 16. The data subjected to the encoding processing are temporarily stored in the transmission buffer 17 and then output through the transmission encoder 18 and an output terminal 19.
Encoded data supplied from a transmission side (not shown) through an input terminal 20 are subjected to decoding processing by the transmission decoder 21. The data subjected to the decoding processing arc temporarily stored in the reception buffer 21 and then subjected to decoding processing by the video-signal multiplexing decoder 23. The data subjected to the decoding processing are subjected to decoding processing by the information-source decoder 24 and then output through an output terminal 25.
FIG. 6 is a block diagram showing an example of the dynamic-picture codec using the video codec (picture compressing unit) 11 shown in FIG. 5. As shown in FIG. 6, in general, a video pre-processor 31 is usually provided at the preceding stage of the video codec 11 as the picture compressing unit 35.
The video pre-processor 31 is formed of a line interpolation circuit 32, a temporal filter 33 and a motion adaptive type spatial filter 34. Then, the line interpolation circuit 32 includes an A-D converter, a memory, a switch for switching input video data and video data read from the memory and so on and repeats one line of the video data at every fifth line to convert the video data of 240 lines (the NTSC system) into the video data of 288 lines (the CIF). That is, in FIG. 4, this unit corresponds to the CIF convening circuit 8.
That is, the input video signal supplied through an input terminal 30 from the signal source is converted into the digital video data by the A-D converter, and the digital video data are stored in the memory. Then, the digital video data from the A-D converter and the digital video data read from the memory are switched by the switch and then output, whereby the video signal formatted in accordance with the NTSC system, the PAL system or the like is converted into the video data formatted in accordance with the CIF.
Also, the temporal filter 33 includes a coefficient ROM, a field memory and an arithmetic circuit and operates as a temporary cyclic type filter for improvement of compression efficiency as pre-processing upon compression of the picture. Then, the motion adaptive type spatial filter 34 includes a block filter, a field memory, a coefficient ROM and an arithmetic circuit and performs the filtering so that motion in the picture after transmitted should become smooth when the picture is displayed on a television monitor or the like, for example.
The video data processed by the video pre-processor 31 explained above are supplied to a picture compressing unit 35 shown in FIG. 5. After being subjected to the above-mentioned processing, the video data are supplied to a multiplexing circuit 36 to be multiplexed. Then, the video data are transmitted through a line interface circuit 37 and an input and output terminal 38 to the other party.
Video data supplied through the input and output terminal 38 from the other party are supplied through the line interface circuit 37 to a separating circuit 39 and subjected to separating processing by the separating circuit 39 to then supply the same to a line thinning-out circuit 40. Then, in order to convert the video data of 288 lines (the CIF) into the video data of 240 lines (the NTSC system), the line thinning-out circuit 40 subjects the input video data to processing to remove one line of video data every sixth line. After conversion into an analog video signal by a D-A converter 41, an output of the line thinning-out circuit 40 is supplied through an output terminal 42 to a display system or the like not shown.
When the signal is convened from that of the NTSC system into that of the CIF, a 5-6 conversion in which one field is used and a 5-3 conversion in which both fields arc used are generally employed. Neither conversion is simple enough to be realized only by one line memory.
In the 5-6 conversion, one line tepee described above causes periodic distortion which drastically deteriorates picture quality and lowers compression efficiency. Even if the picture quality deterioration is permitted, a signal of 288 lines is output while a signal of 240 lines is input. Therefore, different clocks are used in the A-D converter and the codec, so that there is required a circuit for reading date at double speed, which requires additional memory.
Therefore, in general, conversion can be performed by using a field memory. In a simple conversion method, two lines are weighted and added (interpolated) to obtain one line. In a more complicated method, five lines are converted into thirty lines by oversampling to reduce the latter to six lines by a low-pass filter. In either case, a large-capacity memory is required in order to reduce deterioration of the picture quality without lowering the efficiency in the compression of the picture.
As will be clear from the above description, in a codec designed in accordance with the recommendation H. 261, there is a disadvantage that when the television signal of the NTSC system or the PAL system is convened using the CIF, a large-capacity memory is required for a line interpolation circuit of the CIF convening circuit 8 shown in FIG. 4 and the line interpolation circuit 32 of the video pre-processor 31 shown in FIG. 6. Further, its circuit arrangement and its processing process are quite complicated, so that accordingly production costs are high and it is difficult to make the apparatus small in size.
Also, when analog-to-digital conversion is performed by the A-D converter of the above-mentioned line interpolation circuit 32, small phase fluctuations of a phase to be sampled is caused as amplitude fluctuation, which increases an interframe difference in the above-mentioned coder 10. This fact will be explained with reference to FIG. 7.
In FIG. 7, reference letters .DELTA.V represent fluctuation in amplitude caused by the fine fluctuation of the phase to be sampled and the two rows of arrows (.sub.--) represent sampled phases.
If the sampled phases fluctuate as shown by the sampled phase shown by the upper-stage arrow (.sub.--) and the sampled phase shown by the lower-stage arrow (.sub.--), then amplitude fluctuation .DELTA.V results. This appears as the interframe difference in the coder 10.
In a codec operated in accordance with the previously mentioned CCITT recommendation H. 261, the interframe difference is detected and coded. This results in the disadvantage that increase of the interframe difference caused by the amplitude fluctuation .DELTA.V generated from fluctuation of the sampled phases deteriorates the efficiency in the compression of the picture.
The present invention is made in view of such aspects, and an object thereof is to provide the image pickup apparatus which can simplify the circuit arrangement to thereby reduce the production costs and can suppress the increase of the interframe difference caused by the fluctuation of the sampled phases to thereby improve the efficiency in the compression of the picture.